1. Field of the Invention
The invention relates in general to a method of calibrating image luminance values, and more particularly to a method of calibrating the scan luminance value according to an ideal luminance value and a predetermined calibrated luminance value.
2. Description of the Related Art
With development and advancement in science and technology, integration and application of multi-media devices currently have become a trend. Multi-media comprise various data types, such as texts, images, voices, and etc. For data to be presented in the forms of multi-media, several multi-media devices capable of processing images or voices have been invented. In terms of image processing, scanner is one of the most often used image processing devices.
A scanner utilizing conventional image calibration method usually comprises a base, a cover, a platform, a chassis and a white calibration chart. The cover is detachably disposed on the base and can be opened and closed atop the base. The platform is disposed on a top plate of the base for accommodating a document. The white calibration chart is disposed on the inner wall of the top plate of the base and positioned to one side of the platform. The chassis is moveably disposed inside the base and is for scanning the document on the platform and the white calibration chart. Besides, the chassis comprises a light source, a reflector, a lens, an optical sensor, and a charge coupling device (CCD). The light source can be a cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp or a light emitting diode (LED), for instance. The functions of the other elements are disclosed below. Conventionally, to calibrate the luminance value of an image signal outputted by a number of photo-sensing pixels of the optical sensor via the charge coupling device, both of a white calibration and a black calibration of an image signal are required, and illustrated with accompanied drawings.
Referring to FIG. 1, a flowchart of a conventional method of calibrating image luminance values for a scanner is shown. In FIG. 1, firstly, in step 11, execute a white shading correction. The scanner turns on the light source to illuminate the white calibration chart with a light beam. The reflector reflects the reflected light beam by the white calibration chart to the lens. The lens receives and focuses the light to form an image on each pixel of the charge coupling device. Each pixel outputs a corresponding white induced voltage, which is converted into a white actual luminance value by the scanner. Next, in step 12, execute a dark shading correction. When the cover is closed on the base and the scanner turns off the light source, each pixel outputs a corresponding dark induced voltage, which is converted into a dark actual luminance value by the scanner. The dark actual luminance value is smaller than the white actual luminance value.
Then, in step 13, scan the document. Each pixel obtains a corresponding scan inducted voltage, which is converted into a scan luminance value. Next, in step 14, provide a white ideal luminance value and a dark ideal luminance value. The color density of the white calibration chart can be 0.1 for instance. A calorimeter measures luminance value of the white calibration chart and obtains a white ideal luminance value accordingly. After that, in step 15, calibrate the scan luminance value and obtain a calibrated luminance value according to the white actual luminance value, the dark actual luminance value, the white ideal luminance value, and the dark ideal luminance value. Suppose the white actual luminance value, the dark actual luminance value, the white ideal luminance value, the dark ideal luminance value, the scan luminance value and the calibrated luminance value are denoted by VW, VD, TW, TD, VP and OP, then OP=TD+(VP−VD)*[(VP−TD)/(TW−TD)]. VD is also called a dark luminance offset value. (VP−TD)/(TW−TD) is also called a luminance gain value.
When the conventional scanner calibrates the data outputted by the charge coupling device, the luminance of the calibration chart has to maintain uniform, and the distance and angle between the charged coupled device and the calibration chart when the charged coupled device is at white shading correction have to be similar to the distance and angle between the charged coupled device and the document when the charged coupled device is scanning a document. However, after the scanner is used over a period of time, the calibration chart might have non-uniform luminance if the calibration chart comes off the scanner or its coating peels off. Besides, if the distance and angle between the charged coupled device and the calibration chart when the charged coupled device is at white shading correction differ with the distance and angle between the charged coupled device and the document when the charged coupled device is scanning a document, the corresponding white ideal luminance value of a pixel might differ from one to another. Under such circumstance, the conventional method of calibrating scan luminance value would lead to erroneous calibrated luminance values, the image composed of calibrated luminance values would be distorted, largely affecting the scanning quality of a scanner.